This invention relates to electric arc furnaces.
Electric arc furnaces commonly include a hearth, generally vertical side walls and a roof. One or more electrodes extend through suitable openings in the roof for heating the charge contained within the furnace.
Electric furnace steelmaking normally involves the melting of a scrap metal charge which substantially fills the furnace. After charging has been completed, the furnace electrodes are lowered into close proximity with the top of the scrap charge and an arc is struck. Initially, the heat from the arc is directed mainly toward the furnace roof. To minimize roof damage, power is normally maintained at a low level until the electrodes bore into the scrap. At this point, operation is switched to the highest power level for the fastest possible scrap melting. The electrodes melt the scrap charge directly beneath their lower ends and around their outer peripheries as a pool of molten metal collects in the hearth. Upon the formation of the molten metal pool, the charge is melted by the heat from the arc, by the resistance of the current flowing through the charge, and by heat radiated from the molten metal pool and from the electrode. The final process stage is called the "flat bath" condition, which commences when the furnace charge has been entirely melted.
Electric arc furnaces are normally configured to provide an enclosed vessel for containing the materials to be melted and a roof for process containment and thermal efficiency. Originally, arc furnace side walls and roofs were constructed of a refractory material. Also, roofs were formed as an arch so as to be self-supporting. With such refractory lined furnaces, the transition between the second and third melting phases had to be closely controlled to minimize refractory damage which occurs when the furnace lining is exposed to relatively long arcs for a protracted period of time. In order to increase furnace life and enhance efficiency, modern electric arc furnaces normally include water-cooled side walls and roofs.
During a normal scrap melting procedure, the oxidation of carbon, silicon, phosphorus and manganese occur in varying degrees. As a result, clean air regulations require that the furnace be ventilated and that resulting hot gases, dust and fumes be filtered before discharge into the atmosphere. In addition, such regulations place limits on carbon monoxide and nitrogen oxide emissions.
The arched or sloping roof and cylindrical side walls of prior art electric arc furnaces defined a space which conformed roughly with the angle of repose of the scrap and other furnace charge. As a result, such furnaces were not conducive to heat transfer between waste gases and the furnace charge or the oxidation of waste gases passing upwardly from the charge. As a result, relatively large capacity gas cleaning and filtering systems were required.